3GPP (3rd Generation Partnership Project) EPS (Evolved Packet System) is composed of E-UTRAN (Evolved Universal Terrestrial Radio Access Network), MME (Mobility Management Entity), S-GW (Serving Gateway), P-GW (Packet Data Network Gateway), HSS (Home Subscriber Server), 3GPP AAA server (3GPP Authentication, Authorization, and Accounting), PCRF (Policy and Charging Rules Function) and other supporting nodes.
Wherein, MME is responsible for control plane-related work including mobility management, processing of non-access layer signaling, and management of subscriber's mobility management context; S-GW is an access gateway device which is connected with E-UTRAN, and it forwards data between an E-UTRAN and a P-GW and is responsible for buffering paging waiting data. P-GW is a border gateway between 3GPP EPS and a PDN and is responsible for functions such as access of a PDN and data forwarding between an EPS and a PDN.
PCRF is a Policy and Charging Rules Function entity, which interfaces with an operator IP (Internet Protocol) service network through a Rx interface to receive service information; connects with the gateway devices in the network via S7/S7a/S7c interfaces, is responsible for originating establishment of IP bearer, ensures QoS of service data and performs charging control.
FIG. 1 illustrates system architecture of an EPS. As shown in FIG. 1, EPS supports interworking with a non-3GPP network. The interworking with a non-3GPP network is implemented through S2a/b interfaces, and P-GW serves as an anchor between a 3GPP network and a non-3GPP network. In addition, non-3GPP systems are divided into trusted non-3GPP IP access network and untrusted non-3GPP IP access network. Trusted non-3GPP IP access network can directly interface with the P-GW through S2a; untrusted non-3GPP IP access network has to be connected with the P-GW through an ePDG (evolved Packet Data Gateway), and the interface between an ePDG and a P-GW is S2b.
In a conventional 3GPP network, PCEF (Policy and charging enforcement function) only exists in P-GW, so PCRF only needs to interface with P-GW to implement control for all functions. PCRF and P-GW exchange information through a S7 interface. However, when the interface between P-GW and S-GW is based on PMIPv6 (Proxy Mobile IP v6), policy enforcement function of PCEF also partially exists in S-GW, which is referred to as GWCF (Gateway Control Function), also as BBERF (Bearer Binding and Event Reporting Function); a S-GW and a PCRF exchange information through a S7c interface. In the case of accessing through the trusted non-3GPP IP access system, BBERF also resides in the trusted non-3GPP access gateway. A trusted non-3GPP access gateway and a PCRF exchange information through a S7a interface.
When a UE (User Equipment) roams, a S9 interface serves as an interface between PCRF of a home network and PCRF of a visited network; meanwhile AF (Application Function) for providing services for the UE transmits service information for generating a PCC (Policy Charging Control) policy to PCRF through a Rx+ interface.
An EPS system has three types of roaming architectures: one type is called as Home Routed, as shown in FIG. 1, wherein P-GW is in a home network and is provided with IP services by a home network operator (i.e., AF is in the home network); the second type is called as Local Break with home operator's Application Functions only, as shown in FIG. 2; the third type is called as Local Break with visited operator's Application Functions only, as shown in FIG. 3.
There are a plurality of PCRF nodes in one public land mobile network (PLMN) of an EPS, and all of the PCRF nodes belong to one or more Diameter (PCRF) domains (referred to as PCRF domain in short hereinafter), i.e., communication among various PCRF entities are based on Diameter protocol. All PCRFs in one PCRF domain are equal. One connection between the UE and the PDN is called as one IP-CAN (IP Connectivity Access Network) session. A PCC policy of one IP-CAN session is only determined by one PCRF. In order to ensure that all of PCEF, BBERF and AF related with one IP-CAN session are associated to the same PCRF, a logical function module of DRA (Diameter Routing Agent) is incorporated into each PCRF domain in an EPS. When a UE establishes an IP-CAN session to a PDN, the DRA selects a PCRF for this IP-CAN session. PCEF, BBERF and AF related with this IP-CAN session are associated to the selected PCRF through the DRA. However, it is still a problem to be solved as for how a DRA specifically makes an initial selection on PCRF and how to associate PCEF, BBERF and AF related with this IP-CAN session to the same PCRF.